The present invention relates to the use of diffractive gratings for packaging to produce holographic images, and in particular to improving the color quality of such diffractive gratings.
Diffractive devices are used in graphic design for wrapping papers, package covers, labels to authenticate products and many other applications. These devices diffract light into colorful patterns. These diffractive devices or gratings are often created by recording the interference of two laser beams. One of the most popular patterns used by the packaging industries is a so called rainbow hologram which is actually made by the superposition of two linear interference patterns oriented at 90 degrees with respect to each other. More complicated patterns such as those discussed in U.S. Pat. No. 4,568,141, U.S. Pat. No. 5,032,003 are composed of small diffractive units with varying angles. The period inside each diffractive unit is however, constant and may be modified for each diffractive unit to produce a desired effect. The period inside each unit defines a color for that unit, with the multiple units making up a desired pattern. For example, if one wants to observe red color at angle θ from a diffractive unit, the grating periods inside the diffractive unit must be determined by the equations below:
            sin      ⁢                          ⁢      ϑ        =                  λ        r                    T        r              ,where λr and Tr are the wavelength for red color and the corresponding period for the diffractive unit respectively. In recording these diffractive units on a photo-resist recording material, the nonlinear characteristics of the material produce multiple diffractions (second harmonic or sub-harmonic) from the same grating structure. As a result, the light efficiency of the diffractive unit is reduced. Since the diffractive pattern is embossed on a reflective surface, the reduction in the diffractive color will make the pattern appear to be whitish and not colorful.